Recently shift from organic solvent-based compositions to water-based compositions in the art of paint, ink, adhesive and the like is under progress, for conservation of natural resources, environmental sanitation, prevention of pollution and provision of non-dangerous products. For example, as vehicles used in water-based paint compositions, for example, resins such as alkyd resin, acrylic resin, polyester resin, polyurethane resin, epoxy resin and the like can be named. When unsaturated fatty acid is used as a starting material of alkyd resin, oxidation-hardenable groups can be introduced into the resin's skeletal structure, and when such alkyd resin is used as a vehicle of water-based paint compositions, the compositions can be one-liquid cold set. Also due to oiliness of the resin, the water-based paint compositions are expected to exhibit anti-corrosive effect when they are applied onto metallic surfaces. The resin's softness, however, retards drying of the coated film, which generally results in low weatherability. On the other hand, acrylic resin is quick-drying and highly weather-resistant but is inferior in corrosion resistance. As a resinous material for the aqueous system which concurrently exhibits the characteristic properties of these two kinds of resins, graft resins formed by binding alkyd resins and acrylic resins by chemical reaction have been developed and many proposals were made. For example, JP Sho 50 (1975)-126723A, JP Sho 56 (1981)-5863A and Sho 60 (1985)-221469A disclosed a method for producing aqueous solutions or dispersions of such resins, which comprised making a fatty acid-modified monomer through reaction of unsaturated fatty acid having non-conjugated double bond with glycidyl ester of α,β-ethylenically unsaturated acid, copolymerizing the fatty acid-modified monomer with unsaturated monomer such as α,β-ethylenically unsaturated carboxylic acid in organic solvent, neutralizing the carboxyl groups in the resulting resin with basic substance, and thereafter diluting the resin with water. However, those hydrophilic resins according to the method are prepared through solution polymerization, and the formed polymers must be dissolved in organic solvent. Hence it is generally difficult to give them high molecular weight. Furthermore, for dissolving or dispersing the formed polymers in water, monomers having hydrophilic groups such as carboxyl or hydroxyl groups, or an emulsifier must be used in large quantities. In consequence, coating films formed of the polymers are subject to a problem of insufficient water resistance.
JP Sho 59 (1984)-8773A disclosed preparation of an oxidative polymerization type aqueous emulsion by emulsion polymerization of a radical-polymerizable monomeric mixture using a surfactant and/or high molecular protective colloid. The monomeric mixture contained a fatty acid-modified monomer which was obtained through a reaction of a drying oil fatty acid or semi-drying oil fatty acid with glycidyl ester of α,β-etylenically unsaturated acid.
The formation mechanism of an emulsion polymer is such that the participating monomer(s) diffuse in water from large monomer drops present in the aqueous phase, and the polymerization progresses in the micelles formed by surfactant, to produce polymer particles (small particles under polymerization). In that occasion, the monomer(s) used for the polymerization must be supplied under the polymerization conditions, from the monomer drops to the polymer particles, as they diffuse and migrate in the water.
There are generally large differences in particle diameter between the monomer drops and the polymer particles growing from the micelles. This means there is large difference between the total surface area of the monomer drops and that of the polymer particles, and therefore initiator radicals or the monomer(s) diffusing in the water penetrate into those of the greater surface area, i.e., into the polymer particles and the polymerization progresses in situ. During the emulsion polymerization, the polymer particles grow as their diameters gradually increase.
In the occasion of emulsion polymerizing the radical-polymerizable mixture containing the fatty acid-modified monomer, the fatty acid-modified monomer having a very low water-solubility is left in the monomer drops at the polymerization stage, and the polymerization progresses in the monomer drops, producing polymer particles containing many of the fatty acid-modified monomer units. On the other hand, the monomer(s) other than the fatty acid-modified monomer diffuse in the water from the monomer drops into the micelles to form polymer particles containing many units of the monomer(s) other than the fatty acid-modified monomer. Thus, in high probability drastically irregular distribution of hydrophilic polymer particles and hydrophobic polymer particles occurs in the finally obtained emulsion. In consequence, coating films formed from above-described oxidative polymerization type aqueous emulsions have such problems that the hydrophobic polymer particles provide nuclei to cause cissing, or they come up to the surface to retain tackiness at the surface, or due to their incompatibility transparent film cannot be obtained to markedly impair appearance of the finished coating film.
As a measure to overcome those problems as above, EP-A-1044993 proposed aqueous copolymers containing a copolymer which was a polymerization product of vinyl monomers and a hydrophobic ester or partial ester, and their production process. The publication proposed addition of a hydrophobic ester from drying oil acid and polyol to the monomer drops at the polymerization time, to inhibit formation of second generation particles other than the monomer drops. The aqueous copolymers that are described in the publication do not contain volatile fusion assistant and form transparent film, but they are slow drying particularly at the initial stage after application. In actual use the coating film from the copolymers retains tackiness. The coating film is also subject to such defects that its weatherability and water resistance are insufficient.